I got the Bottomless Bubbles on my last cruise. It is definitely worth it. You can also buy a reusable cup if you like. I didn't. I just used the plastic cups they supply. They worked fine. They add the 15% gratuity when you buy the sticker. Just remember, you can only get one glass of soda at a time and the person whose card has the sticker has to get the soda. You can get sodas at any bar and in the MDR.
If you opt to take sodas on board, if you drink all of them and need more, if you are doing the Florida and Bahamas cruise, you can get more when you dock in Port Canaveral. You take take the shuttle to Walmart and get another 12 pack. You can also get sodas in Nassau from several of the shops in The Festival building at the terminal. Unfortunately do not know where you can get sodas in Freeport, but I am sure there is someplace where you can.